1. Field of the Invention
The present invention relates to a sheet shape control method and apparatus, for use in case of changing rolling conditions, which alters sheet dimensions while a sheet is subjected to rolling (hereinafter referred to as a "dimensional alteration in rolling"). Such a case can occur when the same base material is rolled into sheets having various dimensions (including thickness (gauge), width, crown, etc.), i.e., the sheets have different thicknesses and/or widths, and when different types of base materials having different compositions are joined to each other and the joined base materials are rolled continuously.
2. Description of the Related Art
In order to continue processes and improve productivity, techniques of dimensional alteration in rolling for altering dimensions (including thickness, width, crown, etc.) of a sheet material under rolling have been developed in various fields. In the field of cold rolling mills, that technique has already been implemented in many plants. Recently, plants of hot rolling mills have also increasingly employed the dimensional alteration in rolling with the progress of various peripheral techniques.
The dimensional alteration in rolling is performed in the following four cases:
(1) Producing a plurality of sheet products that have a different thickness from a base material having the same composition, PA0 (2) Producing a plurality of sheet products that have a different width from a base material having the same composition, PA0 (3) Producing a plurality of sheet products that have a different width and thickness from a base material having the same composition, and PA0 (4) Joining base materials having different compositions from each other, and rolling the joined base materials continuously. In this case, dimensions and compositions of base materials that are joined to each other may be the same or different.
The dimensional alteration in rolling is practically performed by abruptly changing rolling conditions during rolling, altering a thickness, width, etc. of a rolled sheet, and altering a sheet shape (e.g., roll bending apparatus, roll crossing apparatus, and work roll shifting apparatus). These apparatus are provided in a rolling mill. Accordingly, depending on the control of the sheet shape altering apparatus, a problem arises that the shape of the rolled sheet deteriorates, or an area that includes a shape failure is overly extended in the direction of rolling.
Related art methods for avoiding deterioration of a sheet shape is disclosed in, e.g., Japanese Unexamined Patent Publication Nos. 62-57704 and 4-351213.
Japanese Unexamined Patent Publication No. 62-57704 discloses a method for controlling a shape of a rolled sheet, in, for example, a rolling mill which employs, as sheet shape altering apparatus, a roll bending force, a roll shifting force, and a shift roll. According to the disclosed method, in the case of connecting materials, which are different from each other in thickness, width or both thickness and width, and rolling the connected materials continuously, a mechanical sheet crown model formula is set in advance, which represents a relationship between transverse thickness distribution and rolling conditions resulting when a transverse rolling load acting between the rolled sheet and a work roll is held constant. Using the mechanical sheet crown model formula, or another calculation formula obtained by simplifying and/or modifying the former, the method calculates amounts by which the sheet shape altering apparatus are to be operated in a joined portion between the materials and thereabout. Then, the shape of the sheet under rolling is controlled at a predetermined timing based on the calculated amounts.
Also, the above-cited Japanese Unexamined Patent Publication No. 4-351213 discloses a method for controlling a shape of a rolled sheet by employing, as sheet shape altering apparatus, a roll bending force and a roll cross angle of work rolls, in the case of connecting different types of coils to each other, and rolling the connected coils continuously.
More specifically, as shown in FIG. 2, control of the roll cross angle, which has a slow operating speed, is started toward a target value of the roll cross angle for a succeeding sheet prior to the start of thickness (gauge) alteration. At the same time, adjustment of the roll bending force is also started so as to compensate for the control of the roll cross angle. Then, in synchronism with the thickness alteration, the roll bending force is altered correspondingly with the intended thickness alteration. The control is thus performed so that, at the time when the alteration of the roll cross angle is ended, the roll cross angle and the roll bending force are adjusted to set values for the succeeding sheet.
In any of the above-described methods, an amount of the shape control for a succeeding sheet is estimated before the thickness alteration point reaches a relevant rolling stand, and the amounts by which shape control devices are to be operated are determined based on the estimated amount of the shape control. Therefore, if the target mechanical sheet crown, at the leading end of a succeeding sheet that has been estimated in advance coincides with the actual mechanical sheet crown, a material having been rolled has a satisfactory shape.
In practice, however, a difference, between the target mechanical sheet crown estimated in advance for the leading end of a succeeding sheet, and the actual mechanical sheet crown for the same, may often become substantial, because the actual rolling load fluctuates due to estimation errors of the temperature of a rolled sheet, estimation errors of the resistance to deformation of the rolled sheet, variations in actual thickness, etc. In such an event, the shape control cannot be achieved with a satisfactory level, and inappropriate shape variations occur in a material that has been rolled. Large shape variations raise problems, such as causing the sheet to fracture, and making it difficult to thread the rolled sheet.
The above problems are attributable to the fact that the target mechanical sheet crown is not set during the dimensional alteration in rolling. In other words, an error of the mechanical sheet crown during the dimensional alteration in rolling cannot be evaluated because the target is not set, and the error cannot be corrected by operating the sheet shape altering apparatus.
In the dimensional alteration during rolling, generally, the dimensional alteration is performed in a plurality of rolling stands, with the same point of the rolled sheet set to a start point in order to increase yield. This gives rise to a complicated phenomenon, wherein dimensions of the rolled sheet on both the entry and delivery sides of each rolling stand are altered at the same time.
For the dimensional alteration accompanying such a complicated phenomenon, it has been heretofore considered to be difficult to estimate a mechanical sheet crown, during the dimensional alteration in rolling, with a practically satisfactory level of accuracy, by using a simplified model. On the other hand, computers have been unable to provide for the use of a complex model. For these reasons, it has been customary to only determine the amounts, by which the sheet shape altering apparatus are to be operated, before and after the dimensional alteration in rolling, as with the above-described related art, and setting a target mechanical sheet crown during the dimensional alteration in rolling has been regarded as infeasible.
Further, since hot finish rolling has been heretofore only been applied to rolling steel sheets with a thickness of 1.2 mm or more, no problems have occurred in practical operation, even with conventional methods, in spite of not correcting a shape failure during dimensional alteration from a preceding sheet to a succeeding sheet (i.e., during the dimensional alteration in rolling).
In continuous hot finish rolling which was first performed by Applicants, and in which hot finish rolling is applied to steel sheets with a thickness that is reduced down to 0.8 mm, however, another problem is encountered wherein that fracture of steel sheets occurs unless control, for preventing a shape failure, is continued, even during the dimensional alteration in rolling.
Moreover, Japanese Unexamined Patent Publication No. 59-64111, for example, discloses a method, as one of conventional techniques for controlling a target mechanical sheet crown to be held coincident with an actual mechanical sheet crown during rolling. The disclosed technique is intended to alter an amount of the shape control effected by the sheet shape control apparatus corresponding to a variation in rolling load that is a main cause of variations in mechanical sheet crown.
With the method disclosed in Japanese Unexamined Patent Publication No. 59-64111, however, the target mechanical sheet crown is controlled to be coincident with the actual mechanical sheet crown during rolling, so that the same target mechanical sheet crown is maintained in a single material. Therefore, alteration of the target mechanical sheet crown is not required. By contrast, in the case of rolling materials, that have different dimensions, continuously, as described above, a stable sheet shape is difficult to achieve unless the target mechanical sheet crown is positively altered between a preceding sheet and a succeeding sheet during continuous rolling. Japanese Unexamined Patent Publication No. 59-64111 discloses nothing with regards to a method for altering the target mechanical sheet crown, and hence is difficult to apply to the dimensional alteration in rolling.